The production of particles, such as sand, concerns operators of oil/gas wells because of the possible catastrophic consequences on production. (In this disclosure, “sand should be understood as referring to solid particulate matter as would be found in an oil/gas well, without particular regard to its size or diameter). The production of sand may start at relatively minor levels, but then may rapidly increase resulting in clogged well lines that effectively “fill in” the well and halt production. Sand can also contaminate the separator tanks, which typically connect other producing wells. When this occurs, the production of all oil wells feeding into the separator tanks must be halted. Furthermore, once sand has entered into the completion equipment, corrosion and/or erosion is likely, resulting in significant economic loss.
Operators will thus labor to avoid the production of sand completely, or at least attempt to detect sand at minor levels so that evasive action can be taken. By detecting sand at minor levels the operator may, for example, lower the rate of production (which might allow the sand to fall back through the well), reduce or cease completely any water injection, or in a multiple well system, shut down the affected well completely while allowing the other wells to continue production. In short, the onset of sand production is often the limiting factor in maximizing the production for a given oil and gas well. Because of the serious consequences associated with unnoticed sand production as described above, operators apply conservative production limits, which reduce the maximum production rates. Thus, a large incentive exists in the industry for methods of detecting sand quickly and continuously.
A variety of methods currently exist in the oil and gas industry to detect sand production. One such method is to physically filter a sample of produced fluids to check for solid particles. One problem with this method is that by the time the fluid has risen to the top of the well, it may be too late as contamination of the separator tanks and completion equipment may have already occurred. Furthermore, the filtering of selected samples will not detect sand continuously but instead only at designated time intervals. Therefore, this method is unlikely to detect sand at the inception of production when sand may most likely be encountered.
A technique that continuously monitors for sand production senses the vibrations caused by sand impacting the pipe or conduit in which the sand flows. These devices, such as ClampOn™ meter, clamp on to the pipe, typically at an “elbow” or section of the pipe where the fluid has to take an abrupt turn, and use ultrasonic detection methods to listen for the impact vibration of the sand. However, these ultrasonic methods typically only provide a qualitative measurement and are plagued with the difficulties associated with ultra high frequency coupling into the pipe. Furthermore, the device must be located near an elbow, thus would be unsuitable in the straight or slightly bent piping networks downhole. Although they have the benefit of continuous monitoring, they may also detect the presence of sand too late as they are practically limited to the surface environment.
Real-time monitoring of sand production would be valuable anywhere in the production string, but is particularly valuable downhole, i.e., in conjunction with the production tube, where sand would initially be produced before flowing to the surface. With the emergence of fiber optic sensors, continuous monitoring of fluids in the downhole environment is possible. Fiber optic sensors and flowmeters already monitor parameters such as fluid sound speed, fluid velocity, pressure, and temperature. Such fiber optic based flowmeters are disclosed in the following U.S. patent applications and patents, and are hereby incorporated by reference in their entireties: Ser. No. 09/740,760, entitled “Apparatus for Sensing Fluid in a Pipe,” filed Nov. 29, 2000; Ser. No. 10/115,727, entitled “Flow Rate Measurements Using Unsteady Pressures,” filed Apr. 3, 2002; and U.S. Pat. No. 6,354,147, entitled “Fluid Parameter Measurement in Pipes Using Acoustic Pressures,” issued Mar. 12, 2002 [hereinafter referred to as the “flow meter references.”]. The ability to reliably monitor sand production downhole in real-time, as the above parameters are currently measured, would allow for more effective management of sand production problems. Furthermore, coupling this capability with the real-time measurement of these other parameters results in a powerful fiber optic flowmeter for managing and optimizing well productivity.
The art would therefore benefit from a sensor that can be placed at any location along the production pipe and that can detect sand particles at minimal levels, thus allowing the operator to respond in an appropriate and timely manner to the production of sand.